The aviation industry typically uses two types of radar to track the movement of aircraft.
A primary radar is used to detect the presence of any reflective airborne object. An advantage of this radar is that it detects all reflective aircraft; a disadvantage is that it does not enable the receiver to identify particular aircraft from the received signals.
A secondary radar is used to trigger a reply from transponders (beacons) aboard aircraft. An advantage of this radar is that the transponder provides aircraft identification and elevation information to the receiver; a disadvantage is that only aircraft carrying transponders provide a return signal to the receiver. Fortunately, all commercial and military aircraft, and most privately owned aircraft, now carry transponders, as they are required by the Federal Aviation Administration (FAA) for aircraft entry into controlled airspace at most major airports.
FIG. 1 shows the typical secondary radar signals. In particular, FIGS. 1A and 1B show "mode C" and "mode 3/A" signals transmitted by the ground to the aircraft. FIG. 1C shows a typical transponder signal transmitted when a signal is received from the ground. The distance of the aircraft to the radar transmitter is determined by measuring the time between transmission of a ground signal and receipt of a transponder response. The direction of the aircraft (azimuth) is determined by the direction of the transmitting antenna at the time the highly directional signal is transmitted.
The radar interrogates twice in mode 3/A and once in mode C at a frequency of 347 Hz. The mode 3/A signal asks a transponder to respond with the identity of the aircraft; the mode C signal asks for the elevation of the aircraft. As seen in FIG. 1, each signal has a pretrigger pulse P.sub.t followed by pulses P1, P2, and P3. The duration of each pulse is 0.8 .mu.s, and the time between P.sub.t and P3 is 70 .mu.s for each mode. However, the time between P1 and P3 is 21 .mu.s for mode C and 8 .mu.s for mode 3/A. This difference in time is used by both the transponder and the invention to tell which response is being transmitted from the aircraft. There is approximately 2.9 ms between successive pulses P.sub.t .
The amplitude of pulse P2 is used by the transponder to prevent false responses when the antenna is not pointing at the aircraft. However, this information is not needed by the invention.
FIG. 1C shows the pattern of the transponder response to include 13 equally spaced pulse positions C1 through D4 equally spaced between framing pulses F1 and F2. All pulses in the transponder response have a pulse width of 0.45 .mu.s, and each pulse position is separated from its neighbor by 1.45 .mu.s.
Information is transmitted by the transponder to the ground as a series of octal numbers, A, B, C, D. As indicated in the figure, the pulse positions between F1 and F2 represent values 1, 2, and 4 for each of A through D. FIG. 1D shows a typical response to include pulses F1, A1, C2, A2, B1 and F2. The ground would interpret this information as having values of A =3, B =1, C =2, and D =0. If the information was received in response to a mode 3/A pulse, it would be interpreted as identity information; if it was in response to a mode C pulse, it would be elevation information.
Because the X pulse is not used in response to either mode, and because the D1 pulse is not used in response to mode C, the aforementioned system provides 4096 possible codes for identification and 2048 possible codes for elevation, a sufficient number to provide an indication of height in 100 foot increments between -1000 and +121,000 feet. The information is transmitted in a Gray code to minimize elevation errors due to transmission difficulties.
FAA radar is typically located at large commercial airports. Smaller airports, such as those used by private aircraft, are often located within the field of view of the FAA radar. While it is often desirable for the smaller airport to have access to the information from the FAA radar, because the radar can be receiving responses from a number of aircraft at any one time, the transmission of this amount of information has been quite expensive.